A variety of devices, including sonar devices, radar devices, and lidar devices, that detect targets by transmitting waves such as sound waves, radio waves, and light waves, and receiving reflected waves from the target have been proposed or put to practical use. For these types of target detection devices, a variety of methods for reducing the effects caused by interfering signals (e.g., reverberation) have also been proposed or put to practical use.
For example, according to a proposed technique, which is a first related art of the present invention, reverberations are removed in an active sonar device for detecting moving target objects, by using difference in frequency due to the Doppler effect between a reflected wave from a target object and reverberation (see PTL 1, for example).
According to another proposed technique, which is a second related art of the present invention, reverberations from the bottom or surface of water are separated from reflected waves coming from a target object in an active sonar device for detecting a stationary object, by changing frequency settings for a variable band-stop filter over time (see PTL 2, for example).
According to another proposed technique, which is a third related art of the present invention, an active sonar device called a bi-static or multi-static system where a transmitter and a receiver are located with a certain distance apart, includes a transmitter continuously transmitting an acoustic signal, and a receiver receiving a reflected signal of the acoustic signal reflected from an object and a direct acoustic signal from the transmitter, wherein the receiver performs processing to discriminate the direct acoustic signal from the reflected signal received from the object while the transmitter is transmitting signals (see PTL 3 and PTL 4, for example).
According to another proposed technique, which is a fourth related art of the present invention, in an active sonar device, a transmitter and a receiver are disposed apart from each other by a predetermined distance so as to reduce the effects of acoustic signals from the transmitter (see PTL 5, for example).